The scattering properties of tissue are such that amplitude and instantaneous frequency deviations are induced in the backscattered echo signals. The instantaneous frequency of the ultrasound signal is defined as the time rate of change of the signal's phase. Most conventional ultrasound systems generate images based upon amplitude fluctuations in the ultrasonic waves reflected from the tissue. While the amplitude variations induced by the tissue are useful for determining the boundaries of various body organs and also provide some textural information, instantaneous frequency variations induced by the tissue lead to some frequency demodulated images that may be more useful than amplitude demodulated images in characterizing the textural information.
In U.S. Pat. No. 4,543,826 to the common assignee herein, there is disclosed a system for displaying acoustic images made of an object using said instantaneous frequency deviation information.
Acoustical images are generated by frequency demodulating an electrical signal having frequency deviations corresponding to the scattering properties of a media supporting acoustic pulse propagation against which the signal is projected. The system includes a transmitter which generates a signal at a defined frequency, which signal is then transformed into an acoustic pressure signal and projected against the media by a transducer. The transducer also receives acoustic pressure signals reflected from the media. A receiver coupled to the transducer transforms the reflected acoustic pressure signals into electrical signals which are then frequency demodulated. The demodulated signal contains instantaneous frequency deviations responsive to the scattering properties of the media. The frequency demodulated signal may be processed by circuitry to generate a video signal, which is then algorithmically processed by a scan converter and displayed on a video screen. The frequency demodulation may be done by a conventional demodulator, by a V-demodulator, or any other frequency demodulation process. The frequency demodulated signal may also be combined with an amplitude demodulated image signal. Frequency demodulated and amplitude demodulated signals may be used to provide color images of the media.
Accordingly, it is the primary object of the present invention to improve the quality of the images of a media obtained from a system which uses frequency demodulation of ultrasonic waves to obtain the images.
It is another object of the present invention to provide a method to improve the video output image quality in a system using a frequency demodulation technique to generate acoustical images of the media.
A further object of the present invention is to improve the acoustic images of media utilizing the instantaneous frequency deviations of signals derived from reflected ultrasonic waves.
It is a further object of the present invention to provide an improvement to the acoustical imaging system as disclosed in U.S. Pat. No. 4,543,826 by suppressing the effects of low level signals or noise signals on the video images obtained in the practice of that system.